it的用法精讲
it可用作人称代词、指示代词、先行词及引导词等。
1. 人称代词it，是第三人称单数中性，代表前文已提到过的一件事物。如：
1）That vase is valuable. It's more than 200 years old. 那个花瓶很珍贵，它有200多年的历史。
2）I love swimming. It keeps me fit. 我喜欢游泳，它能使我保持健康。
当说话者不清楚或无必要知道说话对象的性别时，也可用it来表示。 如：
3）It's a lovely baby. Is it a boy or a girl? 宝宝真可爱，是男孩还是女孩？
it可用来指代团体。如：
4）The committee has met and it has rejected the proposal. 委员会已开过会，拒绝了这项建议。
it用以代替指示代词this, that.如：
5）--- What's this? --- It's a pen. —这是什么？ —是一支钢笔。
6）--- Whose book is that? --- It's Mike's. —那是谁的书？ —是迈克的。
2. 指示代词it，常用以指人。如：
7）Go and see who it is. 去看看是谁。
8）--- Who is making such a noise? —是谁发出这样的吵闹声？
--- It must be the children. —一定是孩子们。
3.虚义it无指代性，常用作没有具体意义的主语，出现于表示天气、气候、温度、时间、地点、距离等意义的句子中。如：
9）It is half past three now. 现在是三点半钟。
10）It is six miles to the nearest hospital from here. 这里离最近的医院也有六英里。
11）It was very cold; it snowed and grew dark.
天气很冷；天下着雪，渐渐地变黑了。
it也常用来表示一般的笼统的情况。如：
12）It's awful—I've got so much work I don't know where to start. 糟透了——我有这么多工作要做我不知从何开始。
13）How is it going with you? 你近况如何？
14）Take it easy. 不要紧张。
it也常用于下列结构：
15）It looks as if the college is very small. 看起来这个学院很小。
16）It seems as though our plan will be perfect. 似乎我们的计划很完善。
17）It's my turn. 该轮到我了。
it也常用于某些习惯用语中作宾语，各该习惯用语有具体意义，但it并无具体意义。如：
cab it 乘车 catch it 受责，受罚 come it 尽自己分内 come it strong 做得过分 walk it 步行 make it 办成 take it out of somebody 拿某人出气
4. 先行词it.
it充当形式主语或形式宾语，本身无意义，只起一种先行引导的作用，先行词不重读。后面的真正主语或真正宾语通常是不定式结构、-ing分词结构或名词性从句。
（1）用作形式主语
18）It is difficult to translate this article. 翻译这篇文章很难。
19）It is getting harder every day for a lazy man to get a living. 懒汉谋生是日益困难了。
20）What time would it be most convenient for me to call again?什么时候我再给你打电话最合适？
21）It is no use going there so early. 这么早去那里没有用。
22）It has been a great honour your coming to visit me. 你的来访是我很大的荣幸。
23）It is strange that he did not come at all. 真奇怪，他竟还没来。
24）It doesn't matter what you do.什么都没关系。
在上述诸例中，后置的真正主语可以取代先行it的位置，而出现于句首。
To translate this article is difficult.
Going there so early is no use.
That he did not come at all is strange.
上述结构远不如用先行it结构普通。此外，也并非所有先行it结构都可作上述转换。如：
25）It seems that John is not coming after all. 似乎约翰终竟不来。
26）It happened that John was the only witness. 碰巧约翰是唯一的证人。
上述两例只能转换为：
John doesn't seem to be coming after all.
John happened to be the only witness.
（2）用作形式宾语
27）I found it difficult to explain to him what happened. 我觉得向他解释清发生了什么事很困难。
28）He thought it no use going over the subject again. 他认为再讨论这个问题没有用了。
29）They kept it quiet that he was dead. 他们对他的死保密。
30）We must make it clear to the public that something should be done to stop pollution.我们必须使公众明白应该采取措施制止污染。
分裂句引导词it.
分裂句是以it为引导词的强调句型。它的结构形式是：It+be的一定形式+中心成分+that-/who-从句。如：
31）It was Jane that/who called this morning. 是珍妮今天上午打来电话。
32）It was a parcel that she brought him. 她带给他的是一个包裹。
分裂句是由普通陈述句转换而来的。通过这种结构可以强调除谓语动词以外的大多数句子成分。如：
33）John gave Mary a handbag at Christmas. 约翰在圣诞节给了玛丽一个提包。
根据上下文和语义意图，说话人可以通过分裂句分别强调主语、间接宾语、直接宾语、状语，使之成为信息中心。
34）It was John that/who gave Mary a handbag at Christmas. 是约翰在圣诞节给了玛丽一个提包。
35）It was Mary that John gave a handbag （to） at Christmas. 约翰在圣诞节是给玛丽提包的。
36）It was a handbag that John gave Mary at Christmas. 约翰在圣诞节给玛丽的是一个提包。
37）It was at Christmas that John gave Mary a handbag. 是在圣诞节约翰给了玛丽一个提包。
分裂句的时态一般应一致，即主句与从句的时态应皆用现在时，或皆用过去时，或皆用将来时。如：
38）It is not I who am angry. 发怒的不是我。
39）It was my two sisters who knew her best. 是我的两个姐妹最了解她。
40）It will not be you who will have to take the blame for this. 对此须受责难的将不是你。
有时分裂句的谓语动词还可以采取复杂形式。如：
41）It may have been at Christmas that John gave Mary a handbag. 可能是在圣诞节约翰给玛丽一个提包。
42）It might have been John who gave Mary a handbag. 很可能是约翰给了玛丽一个提包。
分裂句可以强调多种多样的状语成分。如：
43）It was when she was about to go to bed that the telephone rang. 是在她即将上床睡觉时电话铃响了。
44）It was because I wanted to buy a dictionary that I went to town yesterday. 我昨天是由于想买一本词典而进城的。
45）It was not until his father came back that Tom went to bed yesterday.
昨天汤姆是直到他父亲回来才上床睡觉的。（注意句中not的位置）
主语补语通常不可以用作分裂句的中心成分，如在规范英语中通常不说：
It is beautiful that she is.
It is chairman of the Committee that he is.
但是宾语补语是可以这样用的。如：
46）It is the chairman of the committee that they elected him. 他们选他是作委员会主席。
47）It is green that they have painted the wall. 他们把墙壁漆的是绿色。
注：（1）当强调的中心成分为主语，指人时可用连词that或who，如上述例（31）、（34）、（38），强调其余的成分时，连词一律用that。
（2）分裂句中的that与who在非正式文体中可以省略，如：
48）It was the President himself spoke to me.是总统亲自和我谈了话。（省去从句主语who）
49）It was the dog I gave the water to. 我是给那条狗水的。（省去从句的宾语that）
50）It was yesterday I first noticed it. 我是昨天开始注意到的。（省去从句连词that）
有时还可省去句首的it is，如：
51）A good, honest trade you are learning, Sir Peter! 彼得爵士，你学得是一种很好而诚实的一行啊！
（3）分裂句中的被强调部分有时可放在句首，如：
52）Now was it that his life was done, and the fate which he could not escape was upon him.就在这时，他的生命完结了，他所逃不脱的命运降临了。
53）When was it that he arrived at the village? 是何时他到达了这个村子？
54）Why was it that he was late for school? 他是为什么上学迟到的？
1. 分裂句引导词it与先行词it的区别： 比较：
55）It is surprising that Mary should have won the first place. 玛丽本应获得第一名，真令人吃惊！（先行词it）
56）It is Mary that has won the first place. 是玛丽获得了第一名。（分裂句引导词it）
2. 虚义it与分裂句引导词it的区别。 比较：
57）It was three o'clock in the morning when his father came back.（虚义it指时间） 他父亲回来时是凌晨3点。
58）It was at three o'clock in the morning that his father came back. （分裂句引导词it） 他父亲是在凌晨3点回来的。
3. 分裂句中that/who- 从句与that/who引导的定语从句之区别。 比较：
59）It is money that is most needed.（分裂句中的that-从句） 最需要的是钱。
60）This is the money that is most needed.（that引导定语从句） 这就是最需要的钱。
61）It is Mary who has broken the glass （分裂句中的who-从句） 是玛丽打破了玻璃杯。
62）Mary is the girl who has won the first place.（who引导定语从句） 玛丽是获得第一名的姑娘。
区别方法：如将句型It is/was…that结构去掉，而句子能还原成普通陈述句，则该句是分裂句，否则不是。如（56）例去掉It is …that
则变成： Mary has won the first place. 玛丽获得了第一名。
（58）例变成：His father came back at three o'clock in the morning. 他父亲在凌晨3点回来。
（59）例变成：Money is most needed. 钱是最需要的。
（61）例变成：Mary has broken the glass. 玛丽打破了玻璃杯。
4. “拟似”分裂句
如前所述，一般分裂句是不可以用简单陈述句的谓语动词作为中心成分的。如果要强调谓语动词，就得采用另外一种强调句型，这种句型又叫做“拟似”分裂句。如：
63）I gave her a book.
What I did was （to） give her a look. 我所做的是给她一本书。
64）He took a plane to Beijing. What he did was （to） take a plane to Beijing. 他所做的是乘飞机去北京。
65）The student is going to write an article.
What the student is going to do is（to） write an article.这个学生要做的是写一篇文章。
“拟似”分裂句的主语通常是由what引导的名词性从句，从句的主动词通常是do的一定形式；这种分裂句的主语补语通常是不定式结构，可带to，也可不带to，它构成“拟似”分裂句的信息中心。
“拟似”分裂句还可采取另外一种形式，即“what-从句+be+名词词组”的形式，如：
66）What he gave her was a handbag. 他所给她的是个提包。
也可采取“名词词组+be+what-从句”的形式
67）A handbag was what he gave her. 提包就是他所给她的。
巩固练习：
1. 用先行词it改写下列句子：
（1）Possibly the Joneses will come to the party as well.
（2）Whether they will come or not doesn't matter very much.
（3）He is quite likely to let you down.
（4）You seem to have taken a dislike to him.
（5）I happen to have had some nasty experiences in dealing with him.
（6）Getting the car on the boat was rather complicated.
（7）To be with you here this evening is a great pleasure.
（8）You were so kind to invite us.
（9）What you say does not matter in the least.
（10）How he came to have such a valuable painting is a mystery.
2. 翻译下列句子：
（1）冬天，天色晚得早。
（2）问她没有用，她什么也不知道。
（3）从这儿到车站有两英里（路程）。
（4）你见过下冰雹吗？
（5）由你来决定该怎么办？
（6）据说他已去过美国两次了。
（7）可惜你错过了那次音乐会。
（8）看来，他很害怕。
（9）牛奶已经泼翻，哭也无用。（即：作无益的后悔无济于事。）
（10）几个月之后我们才又见面。
3. 用强调句型改写下列句子。（划线部分为中心成分）
（1）She has been studying French only for three months.
（2）He has painted the windows green.
（3）John bought the car from Henry.
（4）I met Henry in New York.
（5）He didn't come home until 11 o'clock last night.
翻译句子答案：
（1）It is getting dark earlier in winter.
（2）It is no use asking her because she didn't know anything about it.
（3）It's two miles from here to the bus station.
（4）Have you seen it hailing?
（5）It is up to you decide what to do.
（6）It is said that he has been to the U.S.A. twice.
（7）It's a pity that you've missed the concert.
（8）It seems that he is much afraid.
（9）It's no use crying over spilt milk.
（10）It was several months before we met again.